Continuous vacuum degassing apparatus with reverse drainage means



Aug. 25, 1970 H. RICHARDSON I 3,

' CONTINUOUS VACUUM DEGASSING APPARATUS WITH REVERSE DRAINAGE MEANSOriginal Filed May 24. 1966 FIG.2

HARRY L. RICHARDSON INVENTOR.

AGENT United States Patent 3,525,510 CONTINUOUS VACUUM DEGASSINGAPPARATUS WITH REVERSE DRAINAGE MEANS Harry L. Richardson, New York,N.Y., assignor to Chemical Construction Corporation, New York, N.Y., acorporation of Delaware Original application May 24, 1966, Ser. No.552,551, now Patent No. 3,457,064, dated July 22, 1969. Divided and thisapplication Mar. 3, 1969, Ser. No. 803,716

Int. Cl. C21c 7/10 U.S. Cl. 266-34 2 Claims ABSTRACT OF THE DISCLOSUREMolten metals are subjected to' vacuum for degassing purposes by flowingthe molten metal stream through a plurality of chambers in the form ofthin horizontal films, with successively higher vacuum or lower absolutepressure being provided in succeeding chambers.

The present application is a division of U.S. patent application No.552,551 filed May 24, 1966 and now U.S. Pat. No. 3,457,064 issued July22,1969.

The present invention relates to the continuous vacuum degassing ofliquids such as molten ferrous metal,

and provides an improved method and apparatus for continuously removinga constituent such as a dissolved impurity from the process liquid as adesorbed gaseous component by the influence of vacuum. The method mayalso be applied as described herein in order to subject the liquid to achemical or physical force or influence which may be varied in order toobtain a change in a characteristic of the liquid as a result of thevacuum atmosphere. This change may consist of physical, chemical,stoichiometric or other natural phenomena and may be caused by chemicalor physical means.

One application of the invention relates to the technique and equipmentto allow molten metals or metallic compounds such as ferrous products,as for instance steel,

to be subjected to an artificial atmosphere in which the absolutepressure is greatly reduced below ambient atmospheric pressure. In theproduction of metals, and particularly in the steel industry, it hasbeen established that a desired improvement in the quality of thefinished product can be materially enhanced by so called vacuumdegassing. This is used as an intermediate step to improve the qualityof any and all types of steel and alloys. The objective of such a stepis the removal of gases which are absorbed, or rhay be generated byreaction of included materials which are deleterious to the desiredphysical characteristics of the finished product.

The established techniques, advantages, and gaseous com-" ponents areWell known in the industry.

The conventional practice to achieve thebenefits derived from the vacuumdegassing of steel involved intermittent, or so-called batch processing,in which a quantity of the semi-finished material, on the order ofperhaps 200 tons, is subjected to an atmosphere which approaches, asnearly as practical limits will allow, an absolute vacuum. This resultsin a reduction of the relative vapor pressure of the gases contained inthe liquid and causes them to be discharged into the evacuated gas areaby well known laws of physics. At present this result is obtained byseveral means, all of which require a vessel to be enclosed in a chamberwhich can be evacuated. This vessel may contain the raw liquid to bedegassed in which case only the surface of the liquid is exposed. Thebulk of the liquid is under ferrostatic pres-.

sure at varying depths below the surface of the liquid and this pressurewill partially or totally overcome the effect ice generated by theevacuated atmosphere depending on the ferrostatic head, which is similarto a hydrostatic fluid head or pressure. With more complex equipment, annon-magnetic or stainless steel vessel may be surrounded by a complexelectrical field to generate induction stirring. Another method instandard practice, as generally illustrated in U.S. Pat. No. 2,937,790,is to place an empty ladle in a chamber which can be evacuated and tointroduce a stream of the liquid into the evacuated chamber, allowing itto fall freely through a limited vertical height before entering thereceiving ladle. This method is known as stream degassing. There areseveral other deviations from these basic procedures, all of whichattempt to secure the exposure of the maximum area of liquid surface tothe evacuated area for the maximum length of time. Typical developmentsin this field include the procedures of U.S. Pat. Nos. 2,997,760;2,893,- 715; 2,882,570; 2,859,262; 2,587,793 and 2,054,923.

The present invention is an improvement based on conclusions relative toknowledge of, and existing technicalfaults of, existing techniques.Thus, the installed and operating costs for maintaining an evacuatedarea, and the removal of gases from this area to maintain thepredetermined absolute pressure, increase in an exponential progressionas the absolute pressure is lowered. In addition, the time required forvacuum removal of the gases from the liquid under ideal conditions isonly an infinitesimal segment of the total time now required for theoverall process. Gaseous equilibrium under ideal conditions is reachedin less than one second in an overall 'treating time of some 35 minutes.Further, sequential atmospheres progressing from ambient to the finaldesired ultimate vacuum are desirable in order to reduce costs and timeof exposure. Finally, a continuous system ten ferrous metal. Theinvention generally involves the provision of a plurality of alignedchambers, which are connected through hydrostatic liquid seals. Theliquid stream flows into each chamber from the previous chamber as athin liquid film and thereafter flows horizontally in the chamber as athin liquid film on a horizontally disposed surface. The liquid flowsfrom the base of the chamber and'thr ough a lower hydrostatic liquidseal to the next succeeding chamber. The chambers are maintained atsub-atmospheric pressures with each succeeding chamber being at a lowerabsolute pressure than the previous chamber. Consequently, impurity isevolved in the gaseous state from'the' thin liquid film within eachchamber and removed due to the vacuum effect and a liquid I stream ofreduced impurity content is removed from the" final chamber. Allevacuating areas are so designed that the hydrostatic head of the liquidduring treatment ap-j proaches zero to the nearest practical limit.

The principal advantage of the present invention is that the desiredobjec'tive of vacuum degassing of a liquid is attained with a minimum ofinstalled equipment and oper-" ating costs. The liquid stream iseffectively dispersed into I horizontally flowing thin liquid films ineach chamber, and thus equilibrium vacuum degassing is eflectively andrapidly attained in each chamber. Another advantage is.

that the initial chambers may be maintained at a vacuum much higher thanthat finally desired. This will allow the bulk of the gasesor gaseousimpurity to be removedat a relatively low operating cost. The liquidstream is thus discharged through any number of succeeding chambers withsteady progression toward the ultimate optimum vacuum atmosphere forfinal removal of small amounts of residual impurity. An added advantageis that the system may be designed to match plant capacity as acontinuous process. Interruptions of the continuous flow are of noconsequence as the vacuum and hydrostatic liquid seals are maintainedwithout liquid flow. In addition, when treating a liquid at elevatedtemperature, the system and chambers are easily preheated with acountercurrent gas fiow before hydrostatic liquid seals are established.The liquid discharge may be easily plugged to allow evacuation of theentire system by the first stage evacuator or vacuum pump, with eachhigher vacuum evacuator or pump being activiated as the hydrostaticliquid seals are established. The hydrostatic liquid seals may beevacuated for complete removal of material from the chambers by areversal of the process, or through external tapholes, if desired.

Each hydrostatic seal also serves as a flotation slag removal skimmer.This feature cannot be incorporated into existing processes. Finally,the relatively small size of the installation allows the use of externalemergency heaters to be incorporated into the system at a very nominalcost.

. It is an object of the present invention to provide an improved methodand apparatus for the continuous vacuum degassing of liquids.

Another object is to remove impurity from a liquid stream in an improvedmanner by vacuum degassing.

A further object is to degass a liquid by disposing the liquid as a thinfilm which is subjected to vacuum.

An additional object is to attain vacuum degassing of a liquid in aplurality of stages within chambers maintained at successively lowerpressure levels and in which the liquid is disposed as a thin film.

Still another object is to degass a liquid in a plurality of connectedchambers in which the liquid is exposed to vacuum as a horizontal filmand a vertical falling film.

An object is to provide an improved method and apparatus for thecontinuous vacuum degassing of molten ferrous metal in a plurality ofconnected chambers maintained at successively lower pressure levels.

These and other objects and advantages of the present invention willbecome evident from the description which follows. The invention will bedescribed relative to a preferred application, consisting of thecontinuous vacuum degassing of molten ferrous metal such as steel.

Referring tothe figures,

FIG. 1 is a sectional elevation view of the apparatus of the invention,and FIG. 2 is a sectional elevation view of FIG. 1, taken on section2-2.

Referring now to FIG. 1, an arrangement involving substantiallyhorizontal flow of the liquid steel as a thin liquid film isillustrated. The molten steel is passed via tundish 76 into an openingin the room 77 of vessel 78, which consists of a suitable molten steelretention vessel such as a ladle. The molten steel flows downwards fromtundish 76 into vessel 78 and joins a pool of molten steel in unit 78,which serves to provide a hydrostatic head for liquid flow through thesystem. A vacuum is maintained within unit 78 by the provision ofsuitable vacuum means as described supra, which serves to removevolatile impurities stream 79 via nozzle 80. Vessel 78 is the first offour vacuum chambers. The gross removal of impurities occurs in thishogging chamber. Gas evolution is usually of such a magnitude that anexplosive etfervescence requires a relatively large area or volume andmay require the provision of spray shields, not shown, adjacent to theinlet of nozzle 80. The liquid flows past lower taphole 81 disposed inthe bottom of unit 78, and next fiows through an opening in the lowerpart of the wall of unit 78. The molten steel thus flows into the firstchamber of the preferably cylindrical and horizontally aligned container82. The first chamber is defined by container 82, the wall of vessel 78,and the substantially vertical partition 83. The

liquid flows upwards into the first chamber past step 84, and then fiowssubstantially horizontally across bottom section as a thin liquid film.A vacuum is maintained within the first chamber by the provision ofsuitable vacuum means as described supra, which serves to removevolatile impurities stream 86 via nozzle 87. The vacuum level is greaterin the first chamber than in the vessel 78, that is, a lower absolutepressure is maintained in the first chamber than in vessel 7 8.

The liquid molten steel next flows under the lower end of partition 83and upwards past the step 88, and into the second chamber definedbetween partition '83 and the substantially vertical partition 89. Theliquid then flows substantially horizontally across bottom section 90 asa thin liquid film. A vacuum is maintained within the second chamber bythe provision of suitable vacuum means as described supra, which servesto remove volatile impurities stream 91 via nozzle 92. The vacuum levelis greater in the second chamber than in the first chamber, that is, alower absolute pressure is maintained in the second chamber than in thefirst chamber.

The liquid molten steel next flows under the lower end of partition 89and upwards past the step 93, and into the third chamber defined betweenpartition 89 and the outlet end 94 of the container 82. The liquid thenflows substantially horizontally across bottom section 95 as a thinliquid film. A vacuum is maintained within the third chamber by theprovision of suitable vacuum means as described supra, which serves toremove volatile impurities stream 96 via nozzle 97. The vacuum level isgreater in the third chamber than in the second chamber, that is, alower absolute pressure is maintained in the third chamher than in thesecond chamber. The third chamber will preferably be maintained at anabsolute pressure of less than 0.015 kg./sq. cm.

The fully degassed and substantially impurity-free liquid now flows fromthe third chamber past level controller 98, and is collected inatmospheric seal leg 99 for subsequent product utilization. Theatmospheric seal leg 99 may consist of a balanced U-seal, or the risingleg of the seal 99 may be truncated to allow the level controller 98 tocontrol the discharge flow of the substantially impurity-free liquid bysuitable means such as a stopper rod, not shown.

The area adjacent and external to seal leg 99 may be flooded with inertgas to prevent recontamination during conventional casting or duringflow into a continuous casting device.

On shut-down of the system, which may be necessitated by a change ofheats or alloy composition, the facility is readily drained free ofresidual molten metal and slag, by the opening of taphole 81. In thiscase, the liquid flow will be reversed, and the residual liquid steeland slag will flow down the steps 93, 88 and 84. The slag originallypresent in the molten steel fed to the tundish 76 is efiectively skimmedoff by the partitions 83 and 89, which also cooperate with the steps 88and 93 respectively to provide hydrostatic liquid seals between thechambers. Residual metal in leg 99 may be removed through an auxiliarytaphole, not shown.

FIG. 2 is a sectional elevation view of the container 82, taken onsection 2-2 of FIG. 1, and shows the preferable cylindrical nature ofthe container 82, as well as the partition 89. FIG. 2 also demonstratesthe minute amount of liquid retained in the vessels to maintaininterchamber seals.

Numerous alternatives within the scope of the present invention willoccur to those skilled in the art. Thus, al though the method andapparatus are particularly applicable to the vacuum degassing of moltensteel, other applications in the degassing or selective removal of aspecific component or impurity from a liquid stream, either at elevatedambient or sub-ambient refrigerated temperatures, will be evident tothose skilled in the art. Additives may be added to the liquid stream ineach of the chambers if desired. While a maximum vacuum or lowestabsolute pressure of less than 0.015 kg./sq. cm. is preferred in thelast and lowest vacuum chamber, a higher pressure for final vacuumdegassing may be pro vided in this chamber in suitable instances. Thenumber of vacuum chambers to be provided in practice, with successivelyreduced absolute pressure levels in succeeding chambers, will depend onthe circumstances of a particular installation. Thus, three vacuumchambers are provided in the assemblage of FIG. 1. In some instances,more than three chambers may be provided, however due to the exposure ofthe liquid to continuous vacuum degassing as a thin horizontal film inthe present invention, rapid impurity removal and equilibrium isattained in each chamber, and consequently three chambers will usuallysuffice in most instances.

An example of an industrial application of the present invention to thecontinuous vacuum degassing of steel will now be described.

EXAMPLE The apparatus of FIG. 1 was designed for a process streamconsisting of 9,000 kg./minute of molten steel at a temperature of 1620C., which was subjected to vacuum degassing as illustrated in FIG. 1.The initial process stream contained dissolved impurities consisting ofap proximately 6.8 ppm. hydrogen, 60 p.p.m. nitrogen and 275 ppm.oxygen. The four vacuum chambers were designed for absolute pressures ofapproximately 0.112, 0.042, 0.014 and 0.0014 kg./ sq. cm. respectively.The four vacuum chambers removed about 60, 20, 8 and 4% of the totaldissolved impurities respectively. The final fully degassed molten steelproduct contained only very minor residual proportions of impurities,and was a high quality finished steel suitable for continuous casting.

I claim:

1. An apparatus for the continuous vacuum removal of an impurity andslag from a molten metal stream containing dissolved impurity and slagwhich comprises a substantially horizontally oriented cylindricalcontainer, at least one substantially vertical partition disposed withinsaid container and serving to divide said container into a plurality ofchambers, said partition terminating above the bottom of said container,means to pass a molten metal stream containing dissolved impurity andslag into an inlet end of said container, whereby said molten metalstream flows substantially horizontally through each of said chambers inseries as a thin molten metal film, and whereby said molten metal streamflows through the opening defined between the lower end of saidpartition and the bottom of said container and thereby maintains ahydrostatic molten metal seal between said chambers, the level of thebottom of said container being successively elevated in stepwiseprogression from the molten metal stream inlet end of said container tothe molten metal stream outlet end of said container, with the lower endof said partition terminating adjacent to and below the top of a step,whereby said hydrostatic molten metal seal is formed between saidchambers and said slag is retained by said partition, means to maintainthe initial molten metal inlet chamber at a sub-atmospheric pressure,means to maintain each succeeding chamber at a lower subatmosphericpressure than the previous chamber, means to remove said molten metalstream of reduced impurity and slag content from the outlet end of saidcontainer, a lower taphole at the inlet end of said container, saidtaphole including valve means adapted to be closed during the flow ofsaid molten metal stream through said chambers, and means to terminatethe flow of said molten metal stream, whereby said taphole valve may beopened by adjustment of said valve means and a reversal of molten metalflow takes place within said chambers to produce reverse drainage flowof substantially all residual molten metal and slag from all of saidchambers through said taphole when the input flow of said molten metalstream is terminated and said taphole valve is opened.

2. An apparatus for the continuous vacuum removal of an impurity andslag from a molten metal stream containing dissolved impurity and slagwhich comprises a plurality of connected cylindrical chambers, saidchambers being juxtaposed in a substantially horizontal plane with eachof said chambers being separated from the next succeeding chamber by amolten metal seal, means to flow a molten metal stream containingdissolved impurity and slag through said plurality of connectedchambers, the first of said chambers being provided with a lowertaphole, said taphole including valve means adapted to be closed duringthe flow of said molten metal stream through said chambers, whereby saidmolten metal stream flows horizontally as a thin molten metal film onthe floor of each chamber, and thereafter flows through said moltenmetal seal to the next succeeding chamber, with the floor of eachsucceeding chamber being elevated above the floor of the previouschamber, whereby said slag is retained in each of said chambers by saidmolten metal seal,

means to maintain a sub-atmospheric pressure level within each chamber,with each succeeding chamber being at a lawor absolute pressure than theprevious chamber, whereby said impurity is evolved from the thin moltenmetal film within each chamber in the gaseous state, means to remove amolten metal stream of reduced impurity and slag content from the finalchamber, said final chamber being maintained at lowest absolutepressure, and means to terminate the flow of said molten metal stream,whereby said taphole valve may be opened by adjustment of said valvemeans and a reversal of molten metal flow takes place within saidchambers to produce reverse drainage flow of substantially all residualmolten metal and slag from all of said chambers through said tapholewhen the input flow of said molten metal stream is terminated and saidtaphole valve is opened.

References Cited UNITED STATES PATENTS Re. 11,737 4/1899 Wainwright.

2,859,262 11/1958 Harders er al 266-34 x 3,193,892 7/1965 Sickbert266-34 X FOREIGN PATENTS 683,996 3/1930 France.

ROBERT D. BALDWIN, Primary Examiner

